Chapter 4 Artificial Radioactive Isotopes

Irene and Frederic Joliot-Curie Chapter 4 Artificial Radioactive Isotopes The Discovery It was mentioned earlier that Irene Joliot-Curie and her husband Frederic Joliot-Curie was awarded the Nobel prize in Chemistry in 1935. The prize was given ”in recognition of their synthesis of new radioactive elements” . Let us see more into the details of this experiment. Irène and Frederic Joliot-Curie had a large supply of polonium, after Irene`s parents. The polonium emitted alpha particles which they used to bombard different elements. In 1933 they used alpha particles and bombarded an aluminum plate. When they removed the α−particle source, it appeared that the aluminum plate emitted radiation with a half-life of approximately 3 minutes. The explanation was that the bombardment had resulted in a nuclear reaction. The α-particle penetrated the aluminum nucleus and changed it into phosphorus by emitting a neutron. The new phosphorus isotope was radioactive and was responsible for the observed radiation. Its designation is P-30. This nuclear reaction may be written as follows: 27 + α ⇒ 30 27 4 30 1 Al P + n or like this 13Al+ 2 He ⇒+ 15 P 0 n Decay of P-30 is: 30 30 15P⇒+ 14 Si positron Energy 3.24 MeV The neutron emitted can be observed as long as the bombardment takes place, but disappears im- mediately when the α-source is removed. However, the phosphorus isotope is radioactive. The decay mode is positron emission as shown above. The half-life is 2.5 minutes. 43 Irene and Fredric Joliot-Curie used their alpha bombardment technique on some other elements and found that it was possible to transform an element into another, with a higher number of protons in its nucleus. They announced this breakthrough to the Academy of Sciences in January of 1934 – and received the Nobel prize the year after. It can be mentioned that Marie Curie died July 1934 and was thus able to follow this exciting research almost up to the third Nobel prize in the family. Radioactive isotopes are made! Radioactive isotopes can be made by bombarding an element with a particle (α-particle, deuteron, proton, electron, neutron and even high energy x-rays). The neutron is the most efficient. In general, the probability of a reaction depends on the energy of the particle – and it is measured in the so called "reaction cross section". It appears that neutrons with low energy (slow neutrons) in general have a larger cross section than fast neutrons. In the mid 1930’s, several laboratories had developed equipment, such as the accelerator called cy- clotron, to bombard stable atoms with protons, deuterons and α-particles and found that new isotopes were formed. Some of these isotopes were radioactive. Since protons, deuterons and α-particles are charged, they will mainly be scattered (because of Cou- lomb interaction) when approaching the positive nucleus of an element. Because of this, and since the techniques available for accelerating particles in the 1930-ties was rather poor, it was impossible (with the available energy) to transform elements with atomic number above 20 . Use of neutrons – discovery of the efficiency of slow neutrons A very efficient particle that can be used for bombardment is the neutron. This particle has no charge and will consequently not be influenced by the electric field around the atomic nucleus. The neutron readily penetrates the atomic nucleus with the result that new isotopes are formed. In Rome, Italy, a small group of excellent scientists (Amaldi, d’Agostino, Pontecorvo, Rasetti, and Segré), headed by Enrico Fermi, used neutrons for bombardment. The neutron source consisted of beryllium powder and radon in a small glass bulb (the α-particles from radon hit beryllium with neutrons as a result). With this relatively strong neutron source they bombarded a number of heavy elements and observed numerous new radioactive isotopes. They assumed that they found both element 93 and 94 (see later). During these experiments Fermi made an important observation. He found that when the source was surrounded by paraffin or water, the efficiency of the neutrons increased by up to 100 times. Conse- quently, when the neutrons were slowed down by collisions with hydrogen atoms, they become more effective. Slow neutrons yield a larger cross-section for many reactions than fast neutrons. With this technique they observed several hundreds new isotopes. These experiments from the 1930-ties earned Fermi the Noble prize in physics for 1938. 44 In 1934 he presented his explanation of the b- decay (a paper that was first not accepted in Na- ture). He then carried out the work with neutrons that earned him the Nobel prize. In USA he was in charge of the work that led to the first controlled nuclear chain reaction – the reactor on a squash court situated beneath Chi- cago's stadium. Today we have several things with his name – Enrico Fermi such as element number 100 which is called Fer- (1901 – 1954) mium, ’’The Fermi National Accelerator Labo- Enrico Fermi was one of the top scientists of the ratory’’ and ’’The Enrico Fermi US Presidential 20th century. He discovered in 1926 the statisti- Award’’, which was started in 1956. cal laws, nowadays known as the «Fermi-Dirac- statistics». In 1927, Fermi became Professor of Theoretical Physics at the University of Rome (a Noble prize in physics 1938 post which he retained until 1938, when he – im- mediately after he received of the Nobel Prize – To Enrico Fermi emigrated to America, primarily to escape Mus- "for his demonstrations of the existence of solini's fascist dictatorship). new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons" Reactors are excellent sources of neutrons and are used for the production of radioactive isotopes needed for biomedical research and the treatment of disease. Also a number of accelerators are used for the formation of particular radioactive isotopes. The number of artificial isotopes increased rapidly in the years after 1934. Thus, by 1937, approximately 200 isotopes were known, in 1949 the number was 650 and today more than 1,300 radioactive isotopes have been produced. Fission A short history James Chadwick discovered the neutron in 1932, and this initiated a number of research projects. The goal was to make and identify the isotopes formed when neutrons penetrate various atomic nuclei. We mentioned above that Fermi used neutrons for this purpose. In 1934 he observed b-particles when he bombarded uranium. Fermi`s interpretation was that he had made a transuranic element – like 93. This interpretation was questioned by the German chemist Ida Noddack. In a famous paper, "Über das Element 93" published in "Zeitschrift fur Angewandte Chemie" in 1934. Noddack suggested a 45 number of possibilities such as "it is conceivable that the nucleus breaks up into several large fragments, which would of course be isotopes of known elements, but would not be neighbors of the irradiated element." In this way Ida Noddack suggested a nuclear fission – which was found a few years later. Since however, she presented no theoretical or chemical basis for this possibil- ity, the paper was more or less ignored at that time. At The Kaiser Wilhelm institute in Berlin Otto Hahn, Lise Meitner and Fritz Strassmann worked with uranium and neutron bombardment. This research Ida Noddack resulted in the conclusion that uranium can be split in two large fragments (1896 – 1978) when bombarded with neutrons (see illustration below). An illustration of a fission 141 56 Ba A slow neutron Neutrons 235 92 U 92 36 Kr Lise Meitner, was an important member of the small group in Berlin. She and Otto Hahn had a lot of discussions in the summer of 1938 – when the political situation in Germany became more and more difficult for Lise Meitner. She was of Jewish ancestry and had lost her Austrian citizenship after the Anschluss and was at great risk. Otto Hahn, Niels Bohr and Dutch physicists were able to help Lise Meitner to escape via the Dutch border and go to Sweden. Here she took up a post at Manne Siegbahn's laboratory. Hahn and Strassmann continued the experiments in Berlin and showed in an experiment on the 17th of December 1938, that it was not possible to separate barium from the compound formed when uranium was hit by neutrons. On the 19th of December, Otto Hahn wrote a letter to Meitner about the latest results. He ended his letter with the sentence: "Perhaps you can Lise Meitner suggest some fantastic explanations". (1878 – 1968) 46 Lise Meitner travelled to Kungälv (just north of Göteborg) to spend the December holidays together with some of her family. Here she met her young nephew Otto Frisch who worked with Bohr. One day they sat on the trunk of a tree, discussing the barium mystery. Using the the nuclear model of Bohr (the so called liquid drop model) as a basis, they calculated that if a neutron penetrated the nucleus, it could set up oscillations that would split the atom - fission was possible ! Maybe it can be said that the atomic age started on a timber log north of Göteborg during the De- cember holidays of 1938. Hahn and Strassmann published their experiments in the German Journal Naturwissenschaften. Meitner and Frisch published their theoretical calculations in the British Journal Nature and Bohr let the news explode at a conference in the United States in January 1939. It was however Otto Hahn – and only him – that was awarded the Noble prize for this discovery. Hahn started to work together with Lise Meitner already in 1907 – and this collaboration lasted for more than thirty years.

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